The present invention relates to a triangular-pyramidal cube-corner retroreflective sheeting having a novel structure. More minutely, the present invention relates to a retroreflective element such as a triangular-pyramidal cube-corner retroreflective element (hereafter merely referred to as a retroreflective element or a reflective element) constituting a retroreflective body useful for reflectors such as signs including traffic signs and construction work signs, license plates of vehicles such as automobiles and motorcycles, safety materials of clothing and life preservers, markings of signboards, and reflectors of visible-light, laser beams, or infrared-ray reflective sensors, and an assembly of the retroreflective elements.
A retroreflective body for reflecting entrance light toward a light source has been well known so far and the reflective body using its retroreflectivity is widely used in the above industrial fields. Particularly, a triangular-pyramidal cube-corner retroreflective body (hereafter also referred to as a CC reflective body) using the internal-total-reflection theory such as a triangular-pyramidal cube-corner retroreflective element (hereafter also merely referred to as a triangular-pyramidal reflective element or CC reflective element) is remarkably superior to a retroreflective body using conventional micro glass beads in retroreflective efficiency of light and thereby, purposes of the triangular-pyramidal cube-corner retroreflective element have been increased year by year because of its superior retroreflective performance.
However, though a conventional publicly-known triangular-pyramidal retroreflective element shows a preferable retroreflective efficiency when an angle formed between the optical axis (axis passing through the apex of a triangle equally separate from three faces constituting a triangular-pyramidal cube-corner retroreflective element and intersecting with each other at an angle of 90xc2x0) of the element and an entrance ray is small because of the reflection theory of the element, the retroreflective efficiency is suddenly lowered (that is, the entrance angle characteristic is deteriorated) as the entrance angle increases. Moreover, the light entering the face of the triangular-pyramidal reflective element at an angle less than the critical angle (xcex1c) meeting an internal-total-reflection condition decided in accordance with the refractive index of a transparent medium constituting the triangular-pyramidal reflective element and that of air reaches the back of the element without totally reflecting from the interface of the element. Therefore, a retroreflective sheeting using a triangular-pyramidal reflective element generally has a disadvantage that it is inferior in entrance angularity.
However, because a triangular-pyramidal retroreflective element can reflect light in the direction in which the light enters over the almost entire surface of the element, reflected light does not reflect by diverging at a wide angle due to spherical aberration like the case of a micro-glass-bead reflective element. However, the narrow divergent angle of the reflected light easily causes a trouble that when the light emitted from a head lamp of an automobile retroreflects from a traffic sign, it does not easily reach, for example, eyes of a driver present at a position separate from the optical axis of the head lamp. The above type of the trouble increases more and more (that is, observation angularity is deteriorated) because an angle (observation angle) formed between the entrance axis of rays and the axis connecting a driver with a reflection point increases.
Many proposals have been made so far for the above cube-corner retroreflective sheeting, particularly for a triangular-pyramidal cube-corner retroreflective sheeting and various improvements are studied.
For example, Jungersen""s U.S. Pat. No. 2,310,790 discloses a retroreflective sheeting constituted by arranging various shapes of retroreflective elements on a thin sheeting and a method for manufacturing the sheeting. The triangular-pyramidal reflective elements disclosed in the above U.S. patent include a triangular-pyramidal reflective element in which the apex is located at the center of a bottom-plane triangle and the optical axis does not tilt (that is, the optical axis is vertical to the bottom plane) and a triangular-pyramidal reflective element in which the apex is not located at the center of a bottom-plane triangle, and it is described in the U.S. patent to efficiently reflect light to an approaching automobile. Moreover, it is described that the depth of a triangular-pyramidal reflective element is kept within {fraction (1/10)} in (2,540 xcexcm). Furthermore, FIG. 15 in the U.S. patent shows a triangular-pyramidal reflective element whose optical axis has a tilt angle (xcex8) of approx. 6.5xc2x0 obtained from the ratio between the major side and the minor side of the bottom-plane triangle of the illustrated triangular-pyramidal reflective element.
However, the above Jungersen""s U.S. patent does not specifically disclose a very-small triangular-pyramidal reflective element disclosed by the present invention or does not describe or suggest a size of a triangular-pyramidal reflective element or a tilt angle the optical axis of the element necessary for superior observation angularity and entrance angularity.
Moreover, in the present specification, the expression xe2x80x9coptical axis tilts in the plus (+) directionxe2x80x9d denotes that the optical axis tilts in the direction in which the difference between the distance (q) from the intersection (Q) of the optical axis of a triangular-pyramidal reflective element and the bottom plane (Sx-Sxxe2x80x2) of the triangular-pyramidal reflective element up to the base edges (x,x, . . . ) shared by the element pair {the distance (q) is equal to the distance from the intersection (Q) up to a plane (Lxxe2x80x94Lx) vertical to the bottom plane (Sx-Sxxe2x80x2) including the bottom edges (x,x, . . . ) shared by the element pair} and the distance (p) from the vertical line extended from the apex of the element to the bottom plane (Sx-Sxxe2x80x2) and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . . ) {the distance (p) is equal to the distance from the intersection (P) up to the vertical plane (Lxxe2x80x94Lx)} becomes plus (+) as described later. On the contrary, when the optical axis tilts in the direction in which (qxe2x88x92p) becomes minus (xe2x88x92), the expression xe2x80x9coptical axis tilts in the direction for the optical axis to become minus (xe2x88x92)xe2x80x9d is displayed.
Moreover, Stamm""s U.S. Pat. No. 3,712,706 discloses a retroreflective sheeting in which the so-called triangular-pyramidal cube-corner retroreflective elements respectively having an equilateral bottom-plane triangle (therefore, the optical axis is vertical to a bottom plane) are arranged on a thin sheeting so that bottom planes of the elements become the closest-packed state on a common plane. In the Stamm""s U.S. patent, means for improving the wide angularity in accordance with the tilt of an optical axis is not described at all.
Furthermore, Hoopman""s European Patent No. 137,736B1 discloses a retroreflective sheeting in which tilted triangular-pyramidal cube-corner retroreflective elements whose bottom-plane triangles are isosceles triangles are arranged on a common plane so that bottom planes of the elements become the closest-packed state. Moreover, it is described that the optical axis of the triangular-pyramidal cube-corner retroreflective element disclosed in the patent tilts in the minus (xe2x88x92) direction and its tilt angle approximately ranges between 7xc2x0 and 13xc2x0.
Furthermore, Szczech""s U.S. Pat. No. 5,138,488 similarly discloses a retroreflective sheeting in which tilted triangular-pyramidal cube-corner retroreflective elements whose bottom-plane triangles are isosceles triangles are arranged on a common plane so that bottom planes of the elements become the closest-packed state. In the U.S. patent, it is specified that the optical axis of a pair of the triangular-pyramidal reflective elements tilts in the direction of the side shared by the triangular-pyramidal reflective element pair faced each other and the tilt angle ranges between approx. 2xc2x0 and 5xc2x0 and each element has a size of 25 to 100 xcexcm.
Moreover, in European Patent No. 548,280B1 corresponding to the above patent, it is described that an optical axis tilts so that the distance (p) between a plane including a side common to two elements which are paired and vertical to a common plan and the apex of an element is not equal to the distance (q) between a point for the optical axis of an element to intersect with the common plane and the vertical plane and the tilt angle ranges between 2xc2x0 and 5xc2x0 and the height of an element ranges between 25 and 100 xcexcm.
As described above, in Szczech""s European Patent No. 548,280B1, the tilt of an optical axis ranges between approx. 2xc2x0 and 5xc2x0 including both plus (+) and minus (xe2x88x92). However, embodiments of the above Szczech""s U.S. patent and European patent only specifically disclose triangular-pyramidal reflective elements in which tilt angles of optical axes are xe2x88x928.2xc2x0, xe2x88x929.2xc2x0, and xe2x88x924.3xc2x0 and the height (h) of an element is 87.5 xcexcm.
The triangular-pyramidal cube-corner retroreflective elements in the above-described conventionally publicly-known Jungersen""s U.S. Pat. No. 2,310,790, Stamm""s U.S. Pat. No. 3,712,706, Hoopman""s European Patent No. 137,736B1, Szczech""s U.S. Pat. No. 5,138,488, and European Patent No. 548,280B1 are common to each other in that bottom planes of many triangular-pyramidal reflective elements serving as cores of entrance and reflection of light are present on the same plane. Moreover, every retroreflective sheeting constituted by triangular-pyramidal reflective elements whose bottom planes are present on the same plane is inferior in entrance angularity, that is, it has a disadvantage that the retroreflectivity suddenly decreases when an entrance angle of light to the triangular-pyramidal reflective element increases.
In general, the following are requested for a triangular-pyramidal cube-corner retroreflective sheeting as basic optical characteristics: high brightness, that is, intensity (magnitude) of reflection brightness represented by the reflection brightness of the light entering from the front of the sheeting, wide angularity, and three performances such as observation angularity, entrance angularity, and rotation angularity about wide angularity.
As described above, every retroreflective sheeting constituted by conventionally publicly-known triangular-pyramidal cube-corner retroreflective elements has a low entrance angularity and its observation angularity can not be satisfied in general. However, the present inventor et al. recently find that it is possible to improve the entrance angularity of a retroreflective sheeting constituted by triangular-pyramidal reflective elements by making the depth (hxe2x80x2) of the element from apexes (H1 and H2) of a face (face c) having one bottom side on the bottom plane (X-Xxe2x80x2) of the triangular-pyramidal reflective element {the depth is equal to the height of apexes (H1 and H2) from the bottom plane (X-Xxe2x80x2)} substantially larger than the depth (h) of a plane (virtual plane Z-Zxe2x80x2) including base edges (z and w) of two faces (face a and face b) substantially perpendicularly crossing the face c of the triangular-pyramidal reflective element from the apex of the plane. The invention of the present inventor et al. is announced in the official gazette No. WO98/18028 internationally released on Apr. 30, 1998.
It is an object of the present invention to provide a triangular-pyramidal cube-corner retroreflective element (CC reflective element) whose entrance angularity and rotation angularity are particularly improved. According to the present invention, the above object and advantage can be achieved by triangular-pyramidal cube-corner retroreflective elements characterized in that the triangular-pyramidal cube-corner retroreflective elements protruded onto a common bottom plane (Sx-Sxxe2x80x2) share a base edge (x) on the bottom plane and are arranged on the bottom plane (Sx-Sxxe2x80x2) in the closest-packed state so as to be faced each other, the bottom plane (Sx-Sxxe2x80x2) is a common plane including many base edges (x,x, . . . ) shared by the triangular-pyramidal reflective elements, two faced triangular-pyramidal reflective elements include the shared base edges (x,x, . . . ) on the bottom plane (Sx-Sxxe2x80x2) and form a pair of substantially-same-shaped elements faced each other so as to be substantially symmetric to planes (Lxxe2x80x94Lx, Lxxe2x80x94Lx, . . . ) vertical to the bottom plane (Sx-Sxxe2x80x2), and the optical axis of the triangular-pyramidal reflective element pair tilts so that the angle formed between the optical axis and a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sx-Sxxe2x80x2) ranges between 0.5xc2x0 and 1.5xc2x0 in the direction in which the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the elements and the distance (p) from the intersection (P) of the vertical line and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . . ) shared by the element pair becomes plus (+) or minus (xe2x88x92).
In the case of the present invention, it is preferable to use a triangular-pyramidal cube-corner retroreflective element in which the optical axis via apexes (H1 and H2) of the above triangular-pyramidal reflective elements tilts by 0.6xc2x0 to 1.4xc2x0 from a vertical line extended from apexes (H1 and H2) of the above triangular-pyramidal reflective elements to the bottom plane (Sx-Sxxe2x80x2) in the direction for (qxe2x88x92p) to become plus (+) or minus (xe2x88x92).
In the case of the present invention, it is more preferable to use a triangular-pyramidal cube-corner retroreflective element in which the optical axis of the triangular-pyramidal reflective elements tilts by 0.6xc2x0 to 1.4xc2x0 in the direction in which the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the elements and the distance (p) from the intersection (P) of a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sx-Sxxe2x80x2) and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the elements becomes plus (+).
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which hx is substantially larger than hy and hz when assuming the height from a bottom plane (Sx-Sxxe2x80x2) including base edges (x,x, . . . ) shared by two triangular-pyramidal reflective elements faced each other up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hx, the height from a bottom plane (Sy-Syxe2x80x2) including the other base edges (y,y, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hy, and the height from a bottom plane (Sz-Szxe2x80x2) including the still other base edges (z,z, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hz.
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which hy and hz are substantially equal to each other and hx is substantially larger than hy and hz when assuming the height from a bottom plane (Sx-Sxxe2x80x2) including base edges (x,x, . . . ) shared by two triangular-pyramidal reflective elements faced each other up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hx, the height from a bottom plane (Sy-Syxe2x80x2) including the other base edges (y,y, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as by, and the height from a bottom plane (Sz-Szxe2x80x2) including the still other base edges (z,z, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hz.
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which the optical axis of the triangular-pyramidal reflective elements tilts by 0.6xc2x0 to 1.4xc2x0 in the direction in which the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the element pair and the distance (p) from the intersection (P) of a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sx-Sxxe2x80x2) and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . . ) shared by the elements becomes minus (xe2x88x92) and moreover, hy and hz are substantially equal to each other and hx is substantially smaller than hy and hz when assuming the height from a bottom plane (Sx-Sxxe2x80x2) including base edges (x,x, . . . ) shared by two triangular-pyramidal reflective elements faced each other up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hx, the height from a bottom plane (Sy-Syxe2x80x2) including the other base edges (y,y, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hy, and the height from a bottom plane (Sz-Szxe2x80x2) including the still other base edges (z,z, . . . ) of the triangular-pyramidal reflective elements up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hz.
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which an inequality xe2x80x9c1.03 less than hmax/hmin less than 1.3xe2x80x9d is satisfied when at least two of the above hx, hy, and hz are substantially different from each other and the maximum one of the hx, hy, and h, is assumed as hmax and the minimum one of them is assumes as hmin.
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which the above hx, hy, and hz respectively range between 50 and 500 xcexcm (both included).
A still-more-preferable triangular-pyramidal cube-corner retroreflective element of the present invention is a triangular-pyramidal cube-corner retroreflective element in which the above hx, hy, and hz respectively range between 60 and 200 xcexcm.
In the case of the present invention, a triangular-pyramidal cube-corner retroreflective element is preferable in which at least one prism-face angle formed when three lateral faces (faces a1, b1, and c1) or (faces a2, b2, and C2) of the triangular-pyramidal cube-corner retroreflective element cross each other ranges between 89.5xc2x0 and 90.5xc2x0 and is slightly deviated from 90.000xc2x0.
In the case of the present invention, a triangular-pyramidal cube-corner retroreflective element is more preferable in which the above triangular-pyramidal cube-corner retroreflective element is sheet-like.
The present invention is described below more minutely.